Fluorescein is a molecule reported in 1871, and has been widely used as a pH indicator or a labeling dye, because of the high water solubility and high fluorescence quantum yield thereof. Since a calcium probe containing fluorescein as a platform was developed, there have been provided a large number of highly sensitive fluorescent off/on type probes utilizing intramolecular photoinduced electron transfer (PeT), decyclization or cyclization of Spiro ring, and the like. In particular, as for probes utilizing the intramolecular photoinduced electron transfer, by designing such probes with taking the oxidation potential of the benzene ring of fluorescein into consideration, off/on of fluorescence can be attained before and after trapping of an object substance for measurement, and an object substance for measurement can be measured with high sensitivity.
There are conventionally known fluorescent probes comprising rhodamine as a platform as fluorescent dye enabling red color bioimaging, and calcium probes such as Rhod-2, and the like have been put into put into practical use as probes utilizing the intramolecular photoinduced electron transfer. However, since rhodamine has amino group in the molecule, it has a problem that it becomes cationic in living bodies, and comes to easily accumulate in specific organelles, especially mitochondria.
Further, almost no reports were made as for structural modification of fluorescein at the oxygen atom of the 10-position of the xanthene ring, and optical characteristics of such compounds wherein the oxygen atom at the 10-position of the xanthene ring is replaced with another type of atom are not known so far. Although a compound corresponding to the basic structure of rhodamine, pyronin Y (PY), of which oxygen atom is replaced with silicon atom (TMDHS) and application of this compound as a fluorescent probe have already been reported (Best, Q et al., Pacifichem 2010, subject number 2335, Dec. 19, 2010; Yuichiro KOIDE et al., Fourth Convention of The Japanese Society for Molecular Imaging, subject number P8-9, May 14, 2009), any compound corresponding to fluorescein of which oxygen atom at the 10-position of the xanthene ring is replaced with silicon atom has not been reported so far, and fluorescent characteristics of such a compound are also not known.